The present invention relates to dispersed particle photoconductive material imaging elements. In particular, this invention relates to imaging element comprising a silver halide emulsion imaging layer which comprises silver halide grains which have not been chemically sensitized and which have deep electron trapping centers.
In conventional silver halide photographic imaging elements, imagewise exposure results in the formation of a xe2x80x9clatent imagexe2x80x9d in exposed silver halide grains, which is subsequently amplified through a photographic development process. The latent image in silver halide crystals is formed through the excitation of free charge carriers by absorbed photons and their subsequent trapping and reaction with interstitial silver ions within the silver halide grain structure to form latent image Agno, centers. Carriers which are thought to play an important role in the formation of latent image centers in silver halide grains are believed to be electrons, holes, and interstitial silver ions. Chemical sensitization of the silver halide grains is typically employed to enable efficient formation of stable latent image centers in the grains upon imagewise exposure. Conventional photographic chemical processing develops silver halide grains having formed latent image centers into silver metal. While the use of silver halide photographic systems employing photographic chemical processing has been widely accepted, in some situations it would be desirable to be able to obtain image data directly from the imagewise exposed material without the need for chemical processing.
Silver halide emulsion grains employed in conventional photographic systems are photoconductors, i.e. when they are exposed, either in the silver halide intrinsic absorption region or in a sensitizing dye absorption region, electrons are excited into the conduction band and these electrons are free to move through the silver halide grain. If these grains are placed in an electromagnetic field and then exposed, this photoconductivity can be detected by measuring the change in the field. The mobility of electrons is far greater than that of holes or interstitial silver ions so that conductivity attributed to photoelectrons is expected to be detectable by measurement of photoconductivity of silver halide grains through use of microwave radiation. Such a measurement has been reported using low temperatures, L. M. Kellogg et al., Photogr. Sci. Eng. Vol. 16, 115 (1972). Experiments designed to detect latent image in silver halide using microwave photoconductivity are given by A. Hasegawa et al., Journal of Imaging Science, Vol. 30, pp. 13-15 (1986). The technique, which is operated at room temperature, is recognized as potentially useful in detection of latent images without the need for conventional chemical development solution processing. However, the use of microwave frequencies to detect latent image in exposed silver halide photographic materials has shown that such photoconductivity is not sufficiently sensitive to detect low exposure levels.
U.S. Pat. No. 4,788,131 discloses a method for electronically processing exposed photographic materials with an improved level of sensitivity for detection and measurement of latent images contained therein. The method includes the steps of placing the element in an electromagnetic field and cooling the element to a temperature between about 4 to about 270K to prevent further image formation; subjecting the element to a uniform exposure of relatively short wavelength radiation; exposing the element to pulsed, high intensity, relatively longer wavelength radiation to excite electrons out of image centers; and measuring any resulting signal with radio frequency photoconductivity apparatus. Shortcomings of this approach, however, are that it needs to be performed at low temperatures, and there is no easy technique disclosed for making a two dimensional scan of the element.
EP 1 139 168 A2 discloses an improved technique for detection and measurement of latent images in silver halide photographic materials by providing a method of electronic processing of a latent image from a photographic element, the method employing pulsed radiation and radio frequency photoconductivity apparatus having a sample capacitor with a gap, that includes the steps of: placing the element in an electromagnetic field adjacent the sample capacitor; providing an advance mechanism for advancing the photographic element past the capacitor; scanning the element through the gap in the sample capacitor with a pulsed, focused beam of radiation; directly measuring the photoelectron response of the element and recording the resulting signals from the radio frequency photoconductivity apparatus; and advancing the element and repeating the exposing and measuring steps to provide a two dimensional readout of the latent image on the photographic element at ambient or lower temperatures. This technique of directly measuring the photoelectron response of the imagewise exposed photographic element to detect the level of exposure the silver halide grains have received is based on the understanding that latent image Agno centers which are formed upon imagewise exposure (when mobile interstitial silver ions in the silver halide grain react with the photoelectrons generated during the exposure) act as electron traps which decrease photoconductivity of exposed silver halide grains. Photoconductivity measured in such process employing photographic elements optimized for formation of Agno latent images thus decreases as the imagewise exposure level the grain has received increases. While the described system is improved relative to the prior art in that there is no need for a uniform exposure of relatively short wavelength radiation (and the associated low temperature cooling step to prevent further image formation) prior to measuring the photoelectron response as well as in providing an easy technique for making a two dimensional scan of the element, photoconductivity measurements obtained by the described process may not be as sensitive as desired in detecting low exposure levels, i.e., giving low photographic speed. Accordingly, it would be desirable to provide an imaging element which may be electronically processed after imagewise exposure to directly measure the photoelectron response of the element with improved sensitivity.
In accordance with a first embodiment of the invention, a photoconductive material imaging element is described comprising a support and a silver halide emulsion imaging layer comprising silver halide grains which have not been chemically sensitized to optimize formation of latent image Agno centers upon imagewise exposure and which are doped with at least 500 deep electron trapping agent dopant centers per grain.
In accordance with a preferred embodiment of the invention, the photoconductive material imaging element includes a planar support and the non-chemically sensitized, deep electron trapping agent doped silver halide grains comprise tabular grains, preferably with an average grain size equivalent circular diameter of greater than 2 xcexcm, with the long dimensions of the tabular grains primarily oriented parallel to the plane of the support.